Problem: Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $y \neq 0$. $p = \dfrac{-3y + 9}{-y - 9} \times \dfrac{y^2 - y - 90}{-4y + 40} $
Explanation: First factor the quadratic. $p = \dfrac{-3y + 9}{-y - 9} \times \dfrac{(y + 9)(y - 10)}{-4y + 40} $ Then factor out any other terms. $p = \dfrac{-3(y - 3)}{-(y + 9)} \times \dfrac{(y + 9)(y - 10)}{-4(y - 10)} $ Then multiply the two numerators and multiply the two denominators. $p = \dfrac{ -3(y - 3) \times (y + 9)(y - 10) } { -(y + 9) \times -4(y - 10) } $ $p = \dfrac{ -3(y - 3)(y + 9)(y - 10)}{ 4(y + 9)(y - 10)} $ Notice that $(y - 10)$ and $(y + 9)$ appear in both the numerator and denominator so we can cancel them. $p = \dfrac{ -3(y - 3)\cancel{(y + 9)}(y - 10)}{ 4\cancel{(y + 9)}(y - 10)} $ We are dividing by $y + 9$ , so $y + 9 \neq 0$ Therefore, $y \neq -9$ $p = \dfrac{ -3(y - 3)\cancel{(y + 9)}\cancel{(y - 10)}}{ 4\cancel{(y + 9)}\cancel{(y - 10)}} $ We are dividing by $y - 10$ , so $y - 10 \neq 0$ Therefore, $y \neq 10$ $p = \dfrac{-3(y - 3)}{4} ; \space y \neq -9 ; \space y \neq 10 $